MEDIUM-ENERGY ELECTRON-DIFFRACTION AND X-RAY PHOTOELECTRON DIFFRACTION STUDY OF PSEUDOMORPHIC FE SILICIDES GROWN ON SI(111) EVIDENCE OF FE VACANCY FORMATION

The structural evolution of metastable Fe silicide layers versus compo sition has been investigated by means of inelastic medium energy elect ron diffraction and photoelectron diffraction. These silicides grown o n Si(111) by Fe and Si co-deposition onto room temperature Si(111) sub strate are found to be epitaxial over a wide composition range from Fe Si to FeSi2. Fe2p(3/2) and Si2p polar scans measured along high symmet ry [1 (2) over bar 1] and [(1) over bar 2 (1) over bar] directions of the substrate as well as inelastic medium energy electron diffraction show that Fe and Si atoms are located in a very similar cubic lattice. Experimental data clearly show the lattice parameter decrease versus Fe content decrease from FeSi to FeSi2. Single scattering calculations have been performed for FeSix silicides with CsCl-type structure. Exp erimental profiles of FeSix (1 < x < 2) are very well reproduced in th e calculations by assuming a CsCl-type FeSix structure in which random ly distributed Fe vacancies are progressively formed when the composit ion evolves from FeSi to FeSi2.